Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 81: CLED agar is preferred to MacConkey agar in the processing of urinary samples because:

A. It supports growth of both Gram-positive and Gram-negative organisms. (Correct Answer)
B. It prevents swarming of Proteus.
C. Growth can be observed within 4 hours.
D. Klebsiella and E.coli can be differentiated easily.

Explanation: ### Explanation **Cystine-Lactose-Electrolyte-Deficient (CLED) Agar** is the standard non-selective culture medium used for the routine processing of urine specimens. **1. Why Option A is Correct:** The primary advantage of CLED over MacConkey agar is its **nutritional versatility**. While MacConkey agar contains bile salts that inhibit most Gram-positive organisms, CLED agar supports the growth of a wide spectrum of urinary pathogens. This includes **Gram-negative bacilli** (like *E. coli*), **Gram-positive cocci** (such as *Staphylococcus saprophyticus*, *Enterococcus*, and *Staphylococcus aureus*), and even **yeasts** (Candida). Since Gram-positive organisms are significant causes of UTIs, CLED ensures they are not missed during primary screening. **2. Analysis of Incorrect Options:** * **Option B:** While CLED **does** prevent the swarming of *Proteus* (due to its electrolyte-deficient nature), this is a shared characteristic with MacConkey agar. Therefore, it is not the *primary* reason CLED is preferred over MacConkey. * **Option C:** Growth on CLED typically requires standard incubation (18–24 hours). It is not a rapid diagnostic medium. * **Option D:** Both CLED and MacConkey use lactose fermentation to differentiate organisms. *E. coli* and *Klebsiella* appear as yellow colonies on CLED and pink colonies on MacConkey; thus, CLED offers no specific diagnostic superiority for this differentiation. **3. Clinical Pearls for NEET-PG:** * **Mechanism:** The "Electrolyte-Deficient" property prevents *Proteus* swarming by limiting the availability of electrolytes required for flagellar movement. * **Indicator:** It uses **Bromothymol Blue** as a pH indicator (Yellow = Lactose Fermenter; Blue/Green = Non-Lactose Fermenter). * **High-Yield Fact:** CLED is ideal for **semi-quantitative colony counts** (using the calibrated loop method) to diagnose significant bacteriuria (Kass criterion: >10⁵ CFU/mL).

Question 82: What test is used for the rapid diagnosis of S.aureus infection?

A. Culture
B. PCR-based assays (Correct Answer)
C. Serological tests
D. ELISA

Explanation: **Explanation:** The correct answer is **PCR-based assays (Option B)** because they offer the fastest turnaround time for identifying *Staphylococcus aureus* and its resistance patterns. **1. Why PCR is correct:** In clinical settings, "rapid diagnosis" refers to results obtained within hours rather than days. PCR (Polymerase Chain Reaction) detects specific DNA sequences of *S. aureus* (e.g., the *nuc* gene) and can simultaneously detect the *mecA* gene to identify Methicillin-resistant *S. aureus* (MRSA). These molecular methods provide results in 1–4 hours, which is critical for initiating appropriate antibiotic therapy in life-threatening conditions like sepsis or endocarditis. **2. Why other options are incorrect:** * **Culture (Option A):** While the "Gold Standard," it is not rapid. It requires 18–24 hours for primary growth and additional time for biochemical confirmation (Catalase/Coagulase tests). * **Serological tests (Option C):** These detect antibodies (e.g., anti-staphylolysin). They are retrospective and unreliable for acute diagnosis because the body takes time to mount an immune response. * **ELISA (Option D):** Generally used for detecting toxins (like TSST-1 or Enterotoxins) rather than the organism itself, and it is less sensitive and slower than PCR for primary diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for MRSA:** Cefoxitin Disc Diffusion test (phenotypic) or PCR for *mecA* gene (genotypic). * **Rapid Biochemical Test:** The **Catalase test** differentiates *Staphylococci* (+) from *Streptococci* (-). * **Confirmatory Test:** The **Tube Coagulase test** is the definitive test for *S. aureus*. * **MALDI-TOF MS:** Another emerging rapid method for identification from positive blood cultures.

Question 83: Which of the following tests is the best for the diagnosis of typhoid fever?

A. Blood culture (Correct Answer)
B. Widal test
C. Stool culture
D. Urine culture

Explanation: **Explanation:** The diagnosis of Typhoid (Enteric) fever depends significantly on the duration of the illness. **Blood culture** is the gold standard and the best diagnostic test during the **first week** of fever, yielding a positive result in approximately 90% of cases. It allows for the definitive isolation of *Salmonella Typhi* and enables antimicrobial susceptibility testing, which is crucial given the rise of multidrug-resistant strains. **Analysis of Options:** * **Widal Test (Option B):** This is a serological test that detects antibodies against O and H antigens. It usually becomes positive only after the **second week** of illness. It is prone to false positives (due to cross-reactivity or prior infection) and false negatives, making it less reliable than cultures. * **Stool Culture (Option C):** This is most useful during the **second and third weeks** of the disease. While it is important for identifying carriers, it has a lower diagnostic yield in the acute early phase compared to blood culture. * **Urine Culture (Option D):** This is typically positive only in the **third and fourth weeks** of the illness and has the lowest diagnostic sensitivity among all culture methods. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic "BASU":** To remember the sequence of positivity: **B**lood (1st week), **A**gglutination/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). * **Bone Marrow Culture:** This is the **most sensitive** test overall (up to 95%), even if the patient has already started antibiotics, but it is not the "best" initial test due to its invasive nature. * **Castaneda’s Medium:** A biphasic medium used for blood cultures to reduce the risk of contamination.

Question 84: A 45-year-old female presents with lower abdominal pain and vaginal discharge. Examination reveals cervicitis and mucopurulent cervical discharge. A Gram smear of the discharge shows abundant pus cells but no bacteria. What is the best approach to isolate the possible causative agent?

A. Culture on chocolate agar supplemented with hemin
B. Culture on McCoy cells (Correct Answer)
C. Culture on a bilayer human blood agar
D. Culture on Vero cell lines

Explanation: **Explanation:** The clinical presentation of cervicitis with mucopurulent discharge and a Gram smear showing "abundant pus cells but no bacteria" (abacterial pyuria/discharge) is a classic description of **Non-Specific Urethritis/Cervicitis**, most commonly caused by ***Chlamydia trachomatis*** (Serotypes D-K). **Why Option B is Correct:** *Chlamydia trachomatis* is an **obligate intracellular bacterium**; it lacks the metabolic machinery to produce its own ATP and therefore cannot be grown on standard artificial (cell-free) culture media like agar. It must be grown in living host cells. **McCoy cells** (mouse fibroblast cell lines) treated with cycloheximide are the gold standard cell line for isolating *Chlamydia*. The cycloheximide inhibits host cell protein synthesis, favoring the growth of the pathogen. **Why Other Options are Incorrect:** * **Option A (Chocolate Agar):** Used for fastidious organisms like *Neisseria gonorrhoeae* and *Haemophilus influenzae*. While *N. gonorrhoeae* also causes cervicitis, it would typically appear as Gram-negative intracellular diplococci on a Gram smear. * **Option C (Bilayer Human Blood Agar):** Specifically used for the cultivation of *Gardnerella vaginalis* (the causative agent of Bacterial Vaginosis). * **Option D (Vero Cell Lines):** These are derived from African green monkey kidney epithelial cells. While used for many viruses (like Polio or Rabies) and certain toxins, they are not the primary choice for *Chlamydia* isolation in a clinical diagnostic setting compared to McCoy cells. **High-Yield Clinical Pearls for NEET-PG:** 1. **Staining:** *Chlamydia* inclusions are best visualized using **Giemsa, Castaneda, or Machiavello stains**. Lugol’s iodine can identify *C. trachomatis* inclusions because they contain glycogen. 2. **Modern Diagnostics:** While culture is specific, **NAAT (Nucleic Acid Amplification Test)** is now the "Investigation of Choice" due to higher sensitivity. 3. **Levinthal’s/Colindale’s Media:** These are other specialized media for *Chlamydia*, but McCoy cells remain the most frequently tested "cell culture" answer.

Question 85: Cerebrospinal fluid (CSF) samples are best stored at which temperature?

A. Room temperature
B. -20°C
C. 4°C (Correct Answer)
D. -70°C

Explanation: **Explanation:** The storage temperature for clinical specimens in microbiology depends entirely on the suspected pathogen and the stability of the sample. For **Cerebrospinal Fluid (CSF)**, the primary goal is to preserve the viability of fastidious organisms while preventing the overgrowth of contaminants. **1. Why 4°C is the Correct Answer:** Standard microbiological practice dictates that if a CSF sample cannot be processed immediately (within 15–30 minutes), it should be **refrigerated at 4°C**. This temperature is optimal for preserving common viral pathogens and preventing the multiplication of contaminating bacteria. * *Note on Bacterial Culture:* While 4°C is the standard for general storage, if **Neisseria meningitidis** or **Haemophilus influenzae** (fastidious, cold-sensitive organisms) are strongly suspected, the sample should ideally be kept at **Room Temperature** or in an incubator (35-37°C) to prevent "cold shock" and loss of viability. However, for general diagnostic protocols and MCQ purposes, 4°C is the accepted standard for short-term storage. **2. Why Other Options are Incorrect:** * **Room Temperature (A):** While better for *N. meningitidis*, it allows rapid multiplication of any contaminating flora, which can lead to false-positive results or obscured cultures. * **-20°C (B):** This temperature causes the formation of large ice crystals that rupture cell membranes and destroy both bacteria and viruses. It is generally avoided in microbiology. * **-70°C (D):** This is used for **long-term preservation** (ultra-low temperature freezing) of viral isolates or bacterial strains in glycerol broth, but it is not appropriate for primary clinical specimens intended for immediate culture. **High-Yield Clinical Pearls for NEET-PG:** * **Golden Rule:** Most specimens (Urine, Sputum, Stool) are stored at **4°C**. * **Exceptions (Do NOT refrigerate):** CSF (if only bacteria are suspected), Blood cultures, and Genital swabs for *N. gonorrhoeae*. * **Viral Transport Media (VTM):** Should always be kept at 4°C during transport. * **CSF Analysis Order:** Tube 1 (Chemistry), Tube 2 (Microbiology), Tube 3 (Hematology/Cell Count) to minimize contamination from the skin or "traumatic tap" blood.

Question 86: Which is the enrichment medium of choice for isolating Salmonella from stool samples during an outbreak of food-borne gastroenteritis?

A. Cary Blair medium
B. V R medium
C. Selenite broth (Correct Answer)
D. Thioglycolate medium

Explanation: ### Explanation The correct answer is **Selenite F broth**. #### 1. Why Selenite Broth is Correct In stool samples, *Salmonella* and *Shigella* are often outnumbered by normal intestinal flora like *E. coli*. To isolate these pathogens, an **enrichment medium** is required. Selenite F broth contains sodium selenite, which inhibits the growth of coliforms (like *E. coli*) and enterococci while allowing *Salmonella* species to multiply. This "enrichment" increases the concentration of the pathogen before subculturing onto solid selective media like DCA or XLD. #### 2. Analysis of Incorrect Options * **Cary Blair Medium (Option A):** This is a **transport medium**, not an enrichment medium. It is used to preserve the viability of pathogens (like *Vibrio cholerae* and *Salmonella*) during transit to the lab but does not actively promote selective multiplication. * **V R Medium (Venkatraman-Ramakrishnan) (Option B):** This is a specialized **transport medium** specifically used for *Vibrio cholerae*. * **Thioglycolate Medium (Option D):** This is a **multipurpose nutrient medium** used primarily to determine the oxygen requirements of microorganisms (aerobes, anaerobes, and facultative anaerobes). It is not selective for enteric pathogens. #### 3. High-Yield Clinical Pearls for NEET-PG * **Enrichment Media for Salmonella:** Selenite F broth and Tetrathionate broth are the gold standards. * **Alkaline Peptone Water (APW):** The enrichment medium of choice for *Vibrio cholerae*. * **Wilson and Blair’s Bismuth Sulfite Agar:** The most sensitive **selective** solid medium for isolating *Salmonella Typhi* (produces characteristic jet-black colonies with a metallic sheen). * **Timing:** For enteric fever (systemic), blood culture is best in the 1st week; for gastroenteritis (localized), stool culture is the primary diagnostic tool.

Question 87: When should the Widal test be performed in a suspected case of typhoid fever?

A. First week of illness
B. Second week of illness (Correct Answer)
C. Third week of illness
D. Fourth week of illness

Explanation: **Explanation:** The Widal test is a serological test that detects agglutinating antibodies (**H and O agglutinins**) against *Salmonella Typhi* and *Salmonella Paratyphi*. The timing of the test is critical for its diagnostic utility. **Why the Second Week is Correct:** Antibodies against the Salmonella antigens start appearing in the serum at the end of the first week, but their titers rise significantly and become detectable by the Widal test only during the **second week** of illness. The titers continue to rise until the third or fourth week, making the second week the earliest and most clinically relevant time to perform the test for a reliable diagnosis. **Analysis of Incorrect Options:** * **First Week:** During this stage, the bacteria are primarily in the bloodstream (**Bacteremia**). The most sensitive diagnostic method here is **Blood Culture** (positive in 90% of cases). Antibody levels are too low to be detected by Widal. * **Third Week:** While the Widal test remains positive, this stage is better characterized by the presence of the organism in the stool and urine. **Stool culture** is most sensitive during the third week. * **Fourth Week:** By this time, the patient is either recovering or entering a chronic carrier state. Antibody titers may begin to decline or remain static, making it less ideal for acute diagnosis compared to the second week. **NEET-PG High-Yield Pearls:** * **Mnemonic for Diagnosis (BASU):** **B**lood culture (1st week), **A**gglutination/Widal (2nd week), **S**tool culture (3rd week), **U**rine culture (4th week). * **Significant Titers:** In India, a titer of **>1:80 for O** and **>1:160 for H** is generally considered significant. * **O vs. H:** 'O' antibodies (somatic) appear early and disappear early (indicate recent infection); 'H' antibodies (flagellar) appear late and persist longer. * **False Positives:** Can occur due to "Anamnestic response" (prior infection or vaccination).

Question 88: True about bacteriuria is:

A. Usually gram positive bacteria in urine
B. Bacterial culture is non-specific
C. More than 1 lac colony forming units/ml of mid-stream urine (Correct Answer)
D. Multiple bacterial species are taken into account

Explanation: ### Explanation **1. Why Option C is Correct:** The diagnosis of a Urinary Tract Infection (UTI) relies on the concept of **Significant Bacteriuria**, first defined by Kass. In an asymptomatic individual, a colony count of **$\geq 10^5$ (1 lakh) colony-forming units (CFU) per ml** of a freshly voided **mid-stream urine (MSU)** sample is the standard diagnostic threshold. This value is used to distinguish true infection from urethral contamination during the voiding process. **2. Why Other Options are Incorrect:** * **Option A:** Most UTIs are caused by **Gram-negative bacilli**, with *Escherichia coli* being the most common pathogen (responsible for ~80% of cases). Gram-positive bacteria like *Staphylococcus saprophyticus* or *Enterococcus* are less frequent. * **Option B:** Bacterial culture is the **gold standard** for diagnosing UTIs. it is highly specific as it identifies the exact causative organism and allows for antimicrobial susceptibility testing (AST). * **Option C:** Significant bacteriuria typically follows the **"Rule of One."** The presence of multiple bacterial species (poly-microbial growth) usually suggests **contamination** of the sample rather than a true infection, unless the patient has a long-term indwelling catheter or a vesicocolic fistula. **3. Clinical Pearls for NEET-PG:** * **Exceptions to the $10^5$ rule:** * **Symptomatic females:** $10^2$ CFU/ml may be significant. * **Suprapubic aspiration:** Any growth (even 1 CFU/ml) is considered significant because the bladder is normally sterile. * **Catheterized patients:** $10^2 - 10^3$ CFU/ml is often significant. * **Preservation:** Urine should be cultured within 1-2 hours. If delayed, it must be refrigerated at **4°C** for up to 24 hours to prevent bacterial overgrowth. * **Screening:** Asymptomatic bacteriuria is only routinely treated in **pregnant women** (to prevent pyelonephritis) and patients undergoing **urological procedures**.

Question 89: Which is the best test to evaluate syphilis after treatment?

A. TPA
B. FTA-ABS
C. TPHA
D. VDRL (Correct Answer)

Explanation: **Explanation:** The evaluation of syphilis treatment relies on monitoring the decline of antibody titers. The correct answer is **VDRL (Venereal Disease Research Laboratory)** because it is a **non-treponemal test**. **1. Why VDRL is the correct answer:** Non-treponemal tests (VDRL and RPR) detect non-specific antibodies (reagin) against cardiolipin-cholesterol-lecithin antigen. These titers correlate with disease activity. Following successful treatment, VDRL titers significantly decline and eventually become negative (seroconversion). A four-fold drop in titer (e.g., from 1:32 to 1:8) is the standard indicator of an adequate therapeutic response. **2. Why other options are incorrect:** * **TPA (Treponema pallidum Agglutination), FTA-ABS (Fluorescent Treponemal Antibody Absorption), and TPHA (T. pallidum Hemagglutination):** These are **treponemal-specific tests**. They detect antibodies against *T. pallidum* itself. Once a person is infected, these tests usually remain positive for life ("treponemal memory"), regardless of successful treatment. Therefore, they cannot distinguish between an active infection and a past, treated infection. **Clinical Pearls for NEET-PG:** * **Screening:** VDRL/RPR (High sensitivity, low specificity). * **Confirmation:** FTA-ABS/TPHA (High specificity). * **Treatment Monitoring:** VDRL (Quantitative titers). * **Neurosyphilis:** CSF-VDRL is the gold standard for diagnosis. * **Prozone Phenomenon:** Can cause a false-negative VDRL in secondary syphilis due to excessively high antibody titers; solved by diluting the serum. * **Biological False Positive (BFP):** Seen in SLE, leprosy, malaria, and pregnancy.

Question 90: What is the grading for tubercle bacilli if the count in an Acid-Fast Bacilli (AFB) sample is greater than 10 per oil immersion field?

A. +
B. ++
C. +++ (Correct Answer)
D. Scanty

Explanation: ### Explanation The grading of Acid-Fast Bacilli (AFB) in sputum smears is standardized by the **Revised National Tuberculosis Control Programme (RNTCP)**, based on the **World Health Organization (WHO)** and **IUATLD** guidelines. This grading system is crucial for assessing the bacterial load, infectivity, and treatment response. **1. Why Option C is Correct:** According to the standard grading for Ziehl-Neelsen (ZN) staining (using a 100x oil immersion objective): * **+++ (3+):** Defined as seeing **>10 AFB per oil immersion field** after examining at least 20 fields. This indicates a very high bacterial load and high infectivity. **2. Analysis of Incorrect Options:** * **Option A (+):** This grade is given when **10–99 AFB are found per 100 oil immersion fields**. * **Option B (++):** This grade is given when **1–10 AFB are found per oil immersion field** (after examining 50 fields). * **Option D (Scanty):** This is used when **1–9 AFB are found per 100 oil immersion fields**. In such cases, the exact number of bacilli must be recorded (e.g., "Scanty 5"). **3. NEET-PG High-Yield Pearls:** * **Minimum Fields:** To declare a slide "Negative," at least **100 oil immersion fields** must be examined without finding any bacilli. * **Sensitivity:** For a smear to be positive, there must be approximately **5,000–10,000 bacilli per ml** of sputum. * **Fluorescence Microscopy:** If using Auramine-O (Fluorescence) staining, the grading differs because the magnification is lower (40x), allowing for a larger field of view. * **Reporting:** Always report the result as "Acid Fast Bacilli seen" rather than "M. tuberculosis seen," as other Nontuberculous Mycobacteria (NTM) can also be acid-fast.

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